1. Field of the Invention
The present invention relates to an automatic brake control unit for controlling an automatic braking device.
2. Description of the Related Art
Conventionally, it is known in the art to provide an automatic braking device that automatically controls the hydraulic pressure of a wheel cylinder independently of the operation of a brake operating member such as a brake pedal by a driver. For example, an automatic braking device described in Japanese Unexamined Patent Application Publication No. 2004-9914 includes a hydraulic pump that can generate pressurization for generating a wheel-cylinder pressure higher than that of a master cylinder that is generated by the operation of the brake operating member, and a normally open linear solenoid valve that can control the pressure applied to master-cylinder pressure using the pressurization by the hydraulic pump independently of the operation of a brake operating member by a driver.
The unit detects the distance between a vehicle equipped with this system and the preceding vehicle, wherein when the detected distance is smaller than a specified reference value, it controls the hydraulic pump and the normally open linear solenoid valve. The unit executes automatic brake control to automatically control a wheel-cylinder pressure using “hydraulic pressure obtained by the addition of the pressurization to the master-cylinder pressure”, thereby automatically applying braking force to the vehicle independently of the operation of a brake operating member by a driver.
In general vehicles, when the driver operates a brake pedal when the automatic brake control is not in operation (hereinafter, this is also referred to as a normal brake operation), the relationship between the stroke of the brake pedal and the wheel-cylinder pressure is indicated by the solid line in FIG. 14. In other words, in the initial stage in which the brake-pedal stroke comes from “0” to a certain value S0, the wheel-cylinder pressure hardly increases even when the brake-pedal stroke increases. Accordingly, a wheel-cylinder pressure that can contribute to the braking of the vehicle cannot be substantially generated in the initial stage.
This is because, in the initial stage, when the wheel-cylinder pressure is “0”, a brake pad moves from a position where it is not in contact with a corresponding brake disc to a position where it is in contact with the brake disc (a position where it can push the brake disc) according to the brake-pedal stroke. That is, most of the brake fluid discharged from the master cylinder according to the brake-pedal stroke is consumed to move the piston in the wheel cylinder with the movement of the brake pad.
Hereinafter, a brake-fluid volume corresponding to the moving distance of the piston in the wheel cylinder with the movement of the brake pad is referred to as “an invalid fluid volume”, and a brake-pedal stroke (the above-mentioned value S0) corresponding to the time that all the invalid fluid volume is discharged from the master cylinder (i.e., the time that a wheel-cylinder pressure that can contribute to the braking of the vehicle substantially starts to be generated) is also referred to as “an invalid stroke”.
In the stage in which the brake-pedal stroke has exceeded the invalid stroke S0, the brake pad is in contact with the brake disc. In this state, the wheel-cylinder pressure that can virtually contribute to the braking of the vehicle increases at a relatively large increase gradient with an increasing in brake-pedal stroke. The increase characteristic of the wheel-cylinder pressure in this stage mainly depends on the operation characteristic of a booster (e.g., a vacuum booster) interposed between the brake pedal and the master cylinder, the elasticity characteristic of the wheel cylinder, the elasticity characteristic of the brake pipe arrangement, and the elasticity characteristic of the brake pad.
Consider the case where a driver starts the operation of the brake pedal in a state in which the foregoing automatic brake control has already been executed, and the wheel-cylinder pressure is maintained at a value at which it can virtually contribute to the braking of the vehicle. Hereinafter, such an operation is also referred to as “a brake override”.
When the brake override is executed, the above-described invalid fluid volume is not consumed to move the piston in the wheel cylinder because the brake-pedal stroke has already been in contact with the brake disc from the state of “0”. Hereinafter, an example will be described in which the amount of pressurization is maintained at value P0 by the automatic brake control, and as a result, the wheel-cylinder pressure is at value P0 at the time of starting a brake-pedal operation (the time of starting a brake override).
In this case, the increase characteristic of the wheel-cylinder pressure corresponding to an increase from the brake-pedal stroke “0” agrees with the increase characteristic of the wheel-cylinder pressure (refer to the part upper than the point z of the solid line in FIG. 14) corresponding to an increase from the value Sz of the brake-pedal stroke in a normal brake operation (refer to FIG. 14).
In other words, the relationship (static relationship) between the brake-pedal stroke and the wheel-cylinder pressure in this case is indicated by the line obtained by translating the part of the solid line in FIG. 14 corresponding to the value P0 of the wheel-cylinder pressure (the part upper than the point z in FIG. 14) to the left in FIG. 14 by the length corresponding to the brake-pedal stroke Sz.
Thus, the relationship between the brake-pedal stroke and the wheel-cylinder pressure varies greatly between that of a normal brake operation (the solid line in FIG. 14) and that of a brake override (the broken line in FIG. 14). More specifically, in the case of the brake override, the wheel-cylinder pressure relative to the same brake-pedal stroke (accordingly, a braking force applied to the vehicle) is significantly higher than that of the normal brake operation.
Thus, the brake feeling of the driver varies greatly between that of the brake override and that of the normal brake operation, posing the problem of generating a brake feeling of significant wrongness during the brake override.